Conformational analysis and vibrational assignments of 2,2,3,3-tetrafluoro-1-propanol CHF2CF2CH2OH as three-rotors system

The conformational stability of 2,2,3,3-tetrafluoro-1-propanol was investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. The calculated potential energy curves of the molecule at DFT-B3LYP level were consistent with five distinct minima that correspond to gauche(-)-gauche-gauche (G1gg), trans-trans-gauche (Ttg), trans-gauche-gauche (Tgg), trans-gauche-gauche(-) (Tgg1) and gauche(-)-gauche-trans (G1gt) conformers in the order of decreasing relative stability. The equilibrium constants for the conformational interconversion of 2,2,3,3-tetrafluoro-1-propanol were calculated and found to correspond to an equilibrium mixture of about 38% G1gg, 28% Ttg, 13% Tgg, 11% Tggt and 10% G1gt conformations at 298.15K. The vibrational frequencies of 2,2,3,3,-tetrafluoro-1-propanol in its five stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.     

Analysis of vibrational spectra of 3-halo-1-propanols CH(2)XCH(2)CH(2)OH (X is Cl and Br)

The conformational stability and the three rotor internal rotations in 3-chloro- and 3-bromo-1-propanols were investigated by DFT-B3LYP/6-311+G and ab initio MP2/6-311+G, MP3/6-311+G and MP4(SDTQ)//MP3/6-311+G levels of theory. On the calculated potential energy surface twelve distinct minima were located all of which were not predicted to have imaginary frequencies at the B3LYP level of theory. The calculated lowest energy minimum in the potential curves of both molecules was predicted to correspond to the Gauche-gauche-trans (Ggt) conformer in excellent agreement with earlier microwave and electron diffraction results. The equilibrium constants for the conformational interconversion of the two 3-halo-1-propanols were calculated at the B3LYP/6-311+G level of calculation and found to correspond to an equilibrium mixture of about 32% Ggt, 18% Ggg1, 13% Tgt, 8% Tgg and 8% Gtt conformations for 3-chloro-1-propanol and 34% Ggt, 15% Tgt, 13% Ggg1, 9% Tgg and 7% Gtt conformations for 3-bromo-1-propanol at 298.15K. The nature of the high energy conformations was verified by carrying out solvent experiments using formamide ( epsilon=109.5) and MP3 and MP4//MP3 calculations. The vibrational frequencies of each molecule in its three most stable forms were computed at the B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecules.     

Three rotor potential energy scans, conformational equilibrium constants and vibrational analysis of 3-fluoro-1-propanol CH(2)FCH(2)CH(2)OH

The conformational stability and the three rotor internal rotations in 3-fluoro-1-propanol were investigated by the DFT-B3LYP/6-311+G** and the ab initio MP2/6-311+G** levels of theory. The calculated potential energy curves of the molecule at both levels of theory were consistent with complex conformational equilibria of about 12 minima, all of which were predicted to have real frequencies at both the B3LYP and the MP2 levels. The lowest energy minimum in the potential curves of 3-fluoro-1-propanol was predicted to correspond to the Gauche-gauche-trans (Ggt) conformer in excellent agreement with microwave and electron diffraction results. The equilibrium constants for the conformational interconversion of the molecule were calculated and found to correspond to an equilibrium mixture of about 33% Ggt, 14% Ggg1 and 13% Gg1g and about 43% Ggt, 12% Ggg1 and 10% Gg1g distribution by the B3LYP/6-311+G** and the MP2/6-311+G** calculations, respectively, at 298.15K. The vibrational frequencies of each molecule in its three stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.     

A study of the conformational stability and the vibrational spectra of 2,3-dichloro-1-propanol

The conformational stability and the three rotor internal rotations in 2,3-dichloro-1-propanol were investigated at DFT-B3LYP/6-311 + G**, MP2/6-311 + G** and MP4(SDQ) levels of theory. From the calculated potential energy surface, ten distinct minima were located all of which were predicted to have real frequencies at the B3LYP level of theory. The calculated lowest energy minima in the potential curves of the molecule were predicted to correspond to the Ggg and Gtg1 structures. The observed broad and very intense infrared band centered at about 3370 cm^?1 supports the existence of the strong intermolecular H-bonding in 2,3-dichloro-1-propanol. The equilibrium constants for the conformational interconversion in the molecule were estimated from the calculated Gibb's energies at the B3LYP/6-311 + G** level of calculation and found to correspond to an equilibrium mixture of about 49% Ggg, 27% Gtg1, 5% Ggt and 5% Tgg conformations at 298.15 K.     

A study of internal rotations and vibrational spectra of oxiranemethanol (glycidol)

The conformational stability and the C–O and O–H internal rotations in oxiranemethanol were investigated at the DFT-B3LYP/6-311G**, MP2/6-311G** and MP4(SDQ)/6-311G** levels of theory. Three minima were predicted in the CCOH potential energy scans of the molecule to have relative energies of about 2 kcal/mol or less and all were calculated to have real frequencies upon full optimization of structural parameters at the DFT and the MP2 levels of calculations. The Cg1 (H bond inner) conformation was predicted to be the lowest energy conformation for oxiranemethanol in excellent agreement with an earlier microwave study. The equilibrium mixture was calculated from Gibb's free-energy changes to be about 79% Cg1, 17% G1g and 3% Gg1 at the B3LYP/6-311G** level and about 87% Cg1, 11% G1g and 2% Gg1 at the MP2/6-311G** level for oxiranemethanol at 298.15 K. No conclusive evidence was obtained for the presence of high-energy form in the liquid phase of oxiranemethanol. The vibrational frequencies of oxiranemethanol in its three stable forms were computed at the B3LYP level and complete vibrational assignments were made for the lowest energy Cg1 form on basis of calculated and experimental data of the molecule.     

Vibrational analyses of vinylsulfonamide CH2CHSO2NH2

The structure and conformational stability of vinylsulfonamide CH2CHSO2NH2 were investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. From the calculations the molecule was predicted to exist predominantly in the gauche-syn (vinyl group nearly eclipses one of the SO bonds and the NH2 and the SO2 moieties eclipse each other) conformation with the possibility of low abundance of the cis-syn and the gauche-anti forms. The asymmetric potential function for the internal rotation about CS bond was determined for the molecule. The vibrational frequencies were computed at DFT-B3LYP level for the gauche-syn conformer of the molecule and its d2(C2H3SO2ND2) and d3(C2D3SO2NH2) deuterated species. Normal coordinate calculations were then carried out and the potential energy distributions were calculated for the molecule.     

An exploration of conformational search of leucine molecule and their vibrational spectra in gas phase using ab initio methods

An extensive exploration of the conformational space has been carried out to characterize all possible gas phase structures of leucine. A total of 324 unique trial structures for canonical leucine were generated by considering all possible combinations of single bond rotamers. All trial structures were optimized at the B3LYP/6-311G* level of the DFT method. A total of 77 unique and stationary canonical conformers were found. Further, 15 most stable conformers were reoptimized at B3LYP/6-311++G** level and their respective relative energies, vertical ionization energies, hydrogen bonding patterns, rotational constants and dipole moments were calculated. A single point energy calculations for leucine conformers have also been done at both B3LYP/6-311++G(2df, p) and MP2/6-311++G(2df, p) levels. The good agreement between our estimates of rotational constants for two most stable conformers and available experimental measurements supports the reliability of the B3LYP/6-311++G** level of theory for describing the conformational behavior of leucine molecule. The proton affinity and gas phase basicity were also determined. Using the statistical approach, conformational distributions at various temperatures have also been performed and analyzed. Vibrational spectra were also calculated. It is also observed that zwitterions of leucine are not stable in gas phase.     

Vibrational analyses of sulfamoyl halides NH2SO2X (X is F, Cl and Br)

The structural stability of sulfamoyl halides NH(2)-SO(2)X (X is F, Cl and Br) were investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. From the calculations the molecules were predicted to exist only in the anti (XS bond is anti with respect to nitrogen lone pair) conformation with the possibility of very low abundance of the syn (SO(2) and NH(2) groups eclipse each other) form of only the fluoride. The equilibrium constant for the syn<-->anti conformational conversion of sulfamoyl fluoride was calculated to be 0.0172 that corresponds to an equilibrium mixture of about 2% syn and 98% anti at 298.15K. The vibrational frequencies were computed at DFT-B3LYP level for the stable anti conformer of the d(0) and d(2) (ND(2)-SO(2)X) deuterated species of the three molecules. Normal coordinate calculations were then carried out and the potential energy distributions were calculated for the molecules.     

Potential energy scans and vibrational assignments of cyclopropanecarboxylic acid and cyclopropanecarboxamide

The structural stability and internal rotations in cyclopropanecarboxylic acid and cyclopropanecarboxamide were investigated by the DFT-B3LYP and the ab initio MP2 calculations using 6-311G** and 6-311+G** basis sets. The computations were extended to the MP4//MP2/6-311G** and CCSD(T)//MP2/6-311G** single-point calculations. From the calculations the molecules were predicted to exist predominantly in the cis (C=O group eclipses the cyclopropane ring) with a cis-trans barrier of about 4-6kcal/mol. The OCOH torsional barrier in the acid was estimated to be about 12-13kcal/mol while the corresponding OCNH torsional barrier in the amide was calculated to be about 20kcal/mol. The equilibrium constant k for the cis<-->trans interconversion in cyclopropanecarboxylic acid was calculated to be 0.1729 at 298.15K that corresponds to an equilibrium mixture of about 85% cis and 15% trans. The vibrational frequencies were computed at the DFT-B3LYP level. Normal coordinate calculations were carried out and potential energy distributions were calculated for the low energy cis conformer of the molecules. Complete vibrational assignments were made on the basis of normal coordinate calculations and comparison with experimental data of the molecules.     

Substituent effects on structural stability of formyl ketene and analysis of vibrational spectra of formyl haloketenes and formyl methylketene.

The conformational behavior and the structural stability of formyl fluoroketene, formyl chloroketene and formyl methylketene were investigated by utilizing quantum mechanical DFT calculations at B3LYP/6-31I + + G** and ab initio calculations at MP2/6-311 + + G** levels. The three molecules were predicted to have a planar s-cis<-->s-trans conformational equilibrium. From the calculations, the direction of the conformational equilibrium was found to be dependent on the nature of the substituting group. In formyl haloketenes, the cis conformation, where the C=O group eclipses the ketenic group, was expected to be of lower energy than the trans conformer. In the case of formyl methylketene the conformational stability was reversed and the trans form (the aldehydic hydrogen eclipsing the ketenic group) was calculated to be about 2 kcal mol(-1) lower in energy than the cis form. The calculated cis-trans energy barrier was found to be in the order: fluoride (15.3 kcal mol(-1)) > chloride (13.1 kcal mol(-1)) > methyl (11.7 kcal mol(-1). Full optimization was performed at the ground and the transition states of the molecules. The vibrational frequencies for the stable conformers of the three ketenic systems were computed at the DFT-B3LYP level, and the zero-point corrections were included into the calculated rotational barriers. Complete vibrational assignments were made on the basis of both normal coordinate calculations and comparison with experimental results of similar molecules.     

Molecular structure of the trans and cis isomers of metal-free phthalocyanine studied by gas-phase electron diffraction and high-level quantum chemical calculations: NH tautomerization and calculated vibrational frequencies

The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.     

Conformational analysis and comparison between theoretical and experimental vibrational spectra for chloroacetyl isocyanate

The conformational stability and vibrational infrared and Raman spectra of chloroacetyl isocyanate (CH2ClCONCO) were investigated by ab initio MP2 and density functional B3LYP calculations using the 6-311 + + G** basis set. From the potential energy scans of the internal rotations of both the halomethyl and the isocyanate rotors, chloroacetyl isocyanate was predicted to exist predominantly in a mixture of the cis-cis (chlorine atom and NCO group eclipse C=O bond) and the gauche-cis (one hydrogen atom and NCO group eclipse C=O bond) conformations with a comparable relative stability. The vibrational wavenumbers of each of the two conformers of the molecule were computed at DFT-B3LYP/6-311 + + G** level. Normal coordinate calculations were carried out to obtain the potential energy distributions (PED) among the symmetry coordinates of the normal modes for each of the stable conformers of chloroacetyl isocyanate. The theoretical vibrational assignments are compared with experimental ones and a ratio of observed/calculated wavenumbers of about 0.97-1.04 was obtained.     

Ab initio studies of aspartic acid conformers in gas phase and in solution

Systematic and extensive conformational searches of aspartic acid in gas phase and in solution have been performed. For the gaseous aspartic acid, a total of 1296 trial canonical structures and 216 trial zwitterionic structures were generated by allowing for all combinations of internal single-bond rotamers. All the trial structures were optimized at the B3LYP/6-311G* level and then subjected to further optimization at the B3LYP/6-311++G** level. A total of 139 canonical conformers were found, but no stable zwitterionic structure was found. The rotational constants, dipole moments, zero-point vibrational energies, harmonic frequencies, and vertical ionization energies of the canonical conformers were determined. Single-point energies were also calculated at the MP2/6-311++G** and CCSD/6-311++G** levels. The equilibrium distributions of the gaseous conformers at various temperatures were calculated. The proton affinity and gas phase basicity were calculated and the results are in excellent agreement with the experiments. The conformations in the solution were studied with different solvation models. The 216 trial zwitterionic structures were first optimized at the B3LYP/6-311G* level using the Onsager self-consistent reaction field model (SCRF) and then optimized at the B3LYP/6-311++G** level using the conductorlike polarized continuum model (CPCM) SCRF theory. A total of 22 zwitterions conformers were found. The gaseous canonical conformers were combined with the CPCM model and optimized at the B3LYP/6-311++G** level. The solvated zwitterionic and canonical structures were further examined by the discrete/SCRF model with one and two water molecules. The incremental solvation of the canonical and zwitterionic structures with up to six water molecules in gas phase was systematically examined. The studies show that combining aspartic acid with at least six water molecules in the gas phase or two water molecules and a SCRF solution model is required to provide qualitatively correct results in the solution.     

Crystal structure and density functional theory study on structural properties and energies of a isonicotinohydrazide compound

An X-ray and a theoretical study of the structure of the isoniazid derivative N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate (1) are reported. In this work, we will report a combined experimental and theoretical study on the molecular structure, vibrational spectra and energies of N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate. The calculated parameters are in good agreement with the corresponding X-ray diffraction values. The FTIR spectrum in the range of 400-4000 cm-1 of N'-(4-dimethylaminobenzylidene)-isonicotinohydrazide monohydrate has been recorded. The molecular geometry and vibrational frequencies and energies in the ground state are calculated by using the DFT (B3LYP, PBE1PBE) methods with 6-311G** basis sets. The calculated HOMO and LUMO energies also confirm that charge transfer occurs within the molecule. The geometries and normal modes of vibrations obtained from B3LYP/PBE1PBE/6-311G** calculations are in good agreement with the experimentally observed data.     

FT-Raman and FT-IR spectra, ab initio and density functional studies of 3,4-dichlorobenzyl alcohol

The Fourier transform Raman and Fourier transform infrared spectra of 3,4-dichlorobenzyl alcohol were recorded in the solid phase. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities, depolarization ratios, reduced masses were calculated by HF and density functional B3LYP method with the 6-311 G** basis set. The scaled theoretical wavenumbers showed very good agreement with the experimental values. The thermodynamic functions of the title compound were also performed at HF/6-311G** and B3LYP/6-311G** levels of theory. A detailed interpretations of the infrared and Raman spectra of 3,4-dichlorobenzyl alcohol is reported. The theoretical spectrograms for FT-IR spectra of the title molecule have been constructed.     

气相中O3与HSO自由基间的氢键复合物

气相中O3与HSO自由基之间的相互作用及其反应在大气化学中非常重要.在DFT-B3LYP/6-311++G**和MP2/6-311++G**水平上求得O3+HSO复合物势能面上的稳定构型,B3LYP方法得到了三种构型(复合物Ⅰ,Ⅱ和Ⅲ),而MP2方法只能得到一种构犁(复合物Ⅱ).在复合物Ⅰ和Ⅲ中,HSO单元中的1H原子作为质子供体.与O3分子中的端基O原子作为质子受体相互作用,形成红移氢键复合物;而在复合物Ⅱ中,虽与复合物Ⅰ和Ⅲ中具有相间的质子供体和质子受体,却形成了蓝移氢键复合物.B3LYP/6-311++G**水平上计算的单体间相互作用能的计算考虑了基组重甍误差(BSSE)和零点振动能(ZPVE)校正,其值在-3.37到-4.55 kJ·mol-1之间.采用自然键轨道理论(NBO)对单体间相互作用的本质进行了考查,并通过分子中原子理论(AIM)分析了三种复合物中氢键的电子密度拓扑性质.     

气相中O3与HSO自由基间的氢键复合物

气相中O3与HSO自由基之间的相互作用及其反应在大气化学中非常重要. 在DFT-B3LYP/6-311++G**和MP2/6-311++G**水平上求得O3+HSO复合物势能面上的稳定构型, B3LYP方法得到了三种构型(复合物I, II和III), 而MP2方法只能得到一种构型(复合物II). 在复合物I和III中, HSO单元中的1H原子作为质子供体, 与O3分子中的端基O原子作为质子受体相互作用, 形成红移氢键复合物; 而在复合物II中, 虽与复合物I和III中具有相同的质子供体和质子受体, 却形成了蓝移氢键复合物. B3LYP/6-311++G**水平上计算的单体间相互作用能的计算考虑了基组重叠误差(BSSE)和零点振动能(ZPVE)校正, 其值在-3.37到-4.55 kJ·mol-1之间. 采用自然键轨道理论(NBO)对单体间相互作用的本质进行了考查, 并通过分子中原子理论(AIM)分析了三种复合物中氢键的电子密度拓扑性质.     

Theoretical determinations of ionization potentials of dopamine

Adiabatic and vertical ionization potentials (IPs) of nine conformers of dopamine in the gas phase are determined using density functional theory (DFT) B3LYP, B3P86, B3PW91 methods and high level ab initio HF method with 6-311++G** basis set, respectively. And the nine stable cationic states have been found in the ionization process of dopamine. Vertical ionization potentials of nine conformers of dopamine are calculated using the older outer-valence Green’s function (OVGF) calculations at 6-311++G** basis set. Vibrational frequencies and infrared spectrum intensities of G1b and G1b⁺ at B3LYP/6-311++G** level are discussed.     

Density functional theory study on the structure and vibrational frequencies of glycylglycine

Eleven possible conformers of glycylglycine have been studied by using the BLYP, B3LYP methods of density functional theory and the HF method at the basis set of 6-311++G**. BLYP (using Becke's and Lee-Yang-Parr's correlation functionals), ab initio Hartree-Fock (HF) and hybrid DFT/HF B3LYP calculations have been carried out to study the structure and vibrational spectra of glycylglycine. Glycylglycine crystal structure has been determined by X-ray diffraction analysis. The title compound has been crystallizes in the orthorhombic space group C1, with Z=4. And the unit cell parameters are: a=8.1184(12)A, b=9.5542(14)A, c=7.8192(11)A and V=577.95(15)A(3). Molecular conformation calculations have got 11 possible conformers. In these possible conformers, the most stable one has been selected. The BLYP/6-311++G** and scaled HF/6-311++G** frequencies correspond well with available experimental assignments of the normal vibrational modes. Comparison of the observed fundamental vibrational frequencies of glycylglycine and calculated results by density functional B3LYP and Hartree-Fock (HF) methods indicates that B3LYP is superior to the scaled Hartree-Fock (HF) for molecular vibrational issues.     

Rotational barriers in monomeric CH2=CX-COOH and CH2=CX-CONH2 (X is H or CH3) and vibrational analysis of methacrylic acid and methacrylamide

The internal rotations in acrylic and methacrylic acids CH2=CX-COOH and their amides CH2=CX-CONH2 (X is H or CH3) were investigated by DFT-B3LYP calculations with 6-311+G** basis set. The potential energy curves were consistent with two minima that correspond to planar cis and trans conformation in the case of the acids (or cis and near-trans forms in the case of the amides). Acrylic acid and acrylamide were predicted to have the cis form as the low and predominant conformation of the molecules. In the case of the methacrylic acid and methacrylamide, the conformational relative stability was predicted to reverse as going from the acrylic to the metha compounds. The trans conformer in methacrylic acid or the near-trans in methacrylamide were predicted to be thermodynamically low energy structures of the molecules. The CCCO rotational barrier was calculated to vary from 4 to 6kcal/mol in the four molecules. The OCOH and OCNH torsional barriers were calculated to be about 13 and 22kcal/mol in the acids and the amides, respectively. The vibrational frequencies of methacrylic acid and methacrylamide were computed at the DFT-B3LYP/6-311+G** level and reliable vibrational assignments were made on the basis of normal coordinate analyses and comparison with experimental data of both molecules in their low energy conformations.